diff --git a/src/core.ts b/src/core.ts
index 70b8e679c..eafddc517 100644
--- a/src/core.ts
+++ b/src/core.ts
@@ -301,18 +301,7 @@ export abstract class APIClient {
       headers[this.idempotencyHeader] = options.idempotencyKey;
     }
 
-    const reqHeaders: Record<string, string> = {
-      ...(contentLength && { 'Content-Length': contentLength }),
-      ...this.defaultHeaders(options),
-      ...headers,
-    };
-    // let builtin fetch set the Content-Type for multipart bodies
-    if (isMultipartBody(options.body) && shimsKind !== 'node') {
-      delete reqHeaders['Content-Type'];
-    }
-
-    // Strip any headers being explicitly omitted with null
-    Object.keys(reqHeaders).forEach((key) => reqHeaders[key] === null && delete reqHeaders[key]);
+    const reqHeaders = this.buildHeaders({ options, headers, contentLength });
 
     const req: RequestInit = {
       method,
@@ -324,9 +313,35 @@ export abstract class APIClient {
       signal: options.signal ?? null,
     };
 
+    return { req, url, timeout };
+  }
+
+  private buildHeaders({
+    options,
+    headers,
+    contentLength,
+  }: {
+    options: FinalRequestOptions;
+    headers: Record<string, string | null | undefined>;
+    contentLength: string | null | undefined;
+  }): Record<string, string> {
+    const reqHeaders: Record<string, string> = {};
+    if (contentLength) {
+      reqHeaders['content-length'] = contentLength;
+    }
+
+    const defaultHeaders = this.defaultHeaders(options);
+    applyHeadersMut(reqHeaders, defaultHeaders);
+    applyHeadersMut(reqHeaders, headers);
+
+    // let builtin fetch set the Content-Type for multipart bodies
+    if (isMultipartBody(options.body) && shimsKind !== 'node') {
+      delete reqHeaders['content-type'];
+    }
+
     this.validateHeaders(reqHeaders, headers);
 
-    return { req, url, timeout };
+    return reqHeaders;
   }
 
   /**
@@ -1013,6 +1028,28 @@ export function hasOwn(obj: Object, key: string): boolean {
   return Object.prototype.hasOwnProperty.call(obj, key);
 }
 
+/**
+ * Copies headers from "newHeaders" onto "targetHeaders",
+ * using lower-case for all properties,
+ * ignoring any keys with undefined values,
+ * and deleting any keys with null values.
+ */
+function applyHeadersMut(targetHeaders: Headers, newHeaders: Headers): void {
+  for (const k in newHeaders) {
+    if (!hasOwn(newHeaders, k)) continue;
+    const lowerKey = k.toLowerCase();
+    if (!lowerKey) continue;
+
+    const val = newHeaders[k];
+
+    if (val === null) {
+      delete targetHeaders[lowerKey];
+    } else if (val !== undefined) {
+      targetHeaders[lowerKey] = val;
+    }
+  }
+}
+
 export function debug(action: string, ...args: any[]) {
   if (typeof process !== 'undefined' && process.env['DEBUG'] === 'true') {
     console.log(`OpenAI:DEBUG:${action}`, ...args);
diff --git a/tests/index.test.ts b/tests/index.test.ts
index 538f7dfc9..e056d3b85 100644
--- a/tests/index.test.ts
+++ b/tests/index.test.ts
@@ -28,25 +28,25 @@ describe('instantiate client', () => {
 
     test('they are used in the request', () => {
       const { req } = client.buildRequest({ path: '/foo', method: 'post' });
-      expect((req.headers as Headers)['X-My-Default-Header']).toEqual('2');
+      expect((req.headers as Headers)['x-my-default-header']).toEqual('2');
     });
 
-    test('can be overriden with `undefined`', () => {
+    test('can ignore `undefined` and leave the default', () => {
       const { req } = client.buildRequest({
         path: '/foo',
         method: 'post',
         headers: { 'X-My-Default-Header': undefined },
       });
-      expect((req.headers as Headers)['X-My-Default-Header']).toBeUndefined();
+      expect((req.headers as Headers)['x-my-default-header']).toEqual('2');
     });
 
-    test('can be overriden with `null`', () => {
+    test('can be removed with `null`', () => {
       const { req } = client.buildRequest({
         path: '/foo',
         method: 'post',
         headers: { 'X-My-Default-Header': null },
       });
-      expect((req.headers as Headers)['X-My-Default-Header']).toBeUndefined();
+      expect(req.headers as Headers).not.toHaveProperty('x-my-default-header');
     });
   });
 
@@ -179,12 +179,27 @@ describe('request building', () => {
   describe('Content-Length', () => {
     test('handles multi-byte characters', () => {
       const { req } = client.buildRequest({ path: '/foo', method: 'post', body: { value: '—' } });
-      expect((req.headers as Record<string, string>)['Content-Length']).toEqual('20');
+      expect((req.headers as Record<string, string>)['content-length']).toEqual('20');
     });
 
     test('handles standard characters', () => {
       const { req } = client.buildRequest({ path: '/foo', method: 'post', body: { value: 'hello' } });
-      expect((req.headers as Record<string, string>)['Content-Length']).toEqual('22');
+      expect((req.headers as Record<string, string>)['content-length']).toEqual('22');
+    });
+  });
+
+  describe('custom headers', () => {
+    test('handles undefined', () => {
+      const { req } = client.buildRequest({
+        path: '/foo',
+        method: 'post',
+        body: { value: 'hello' },
+        headers: { 'X-Foo': 'baz', 'x-foo': 'bar', 'x-Foo': undefined, 'x-baz': 'bam', 'X-Baz': null },
+      });
+      expect((req.headers as Record<string, string>)['x-foo']).toEqual('bar');
+      expect((req.headers as Record<string, string>)['x-Foo']).toEqual(undefined);
+      expect((req.headers as Record<string, string>)['X-Foo']).toEqual(undefined);
+      expect((req.headers as Record<string, string>)['x-baz']).toEqual(undefined);
     });
   });
 });